We did some research on this topic and found the following web pages:
https://www.caliandro.de/blog/android-probleme-mit-konten/ (german)

These pages describe similar cases, especially in relation with Android 7.1.1. Android 7.1.1 produces in combination with system-tools, for example systems for backups, defective accounts.

Possible Solutions:
1. In the Onleihe-app go to "My account", if you can see your account there, please try to delete the account.
=> click on the "3 dots"- menu and "Delete account". After this, try the normal process of adding your account and logging in.

2. In Android Settings -> General -> Accounts -> Onleihe - the precise location may vary with each device / manufacturer - if you can see the Onleihe account there, delete it.
After deleting the account in the Settings, try try the normal process of adding your account in the Onleihe app and logging in.

3. The last possible solution is to get a new account from your library. We received feedback that this was a solution for some library customers.
This solution should only be tried if 1. and 2. did not solve the issue.

At this time, we do not know of any other solutions for this issue.